Mutations of myosin and actin can produce familial hypertrophic cardiomyopathy (FHC) or dilated cardiomyopathy (DCM), diseases of the sarcomere where the resulting hypertrophy or dilation is associated with high rates of mortality and morbidity. In this study we use genetically-altered mice that mimic some of the key point mutations found in FHC (myosin: R453C, G741R, R403Q; actin: E99K, A331P, A295S) and DCM (myosin: S532P, F764L; actin R312H, E361G). Isolated myofibrils and permeabilized muscle strips will be studied mechanically to provide a detailed functional characterization of mutant contractile proteins within the spatial constraints of the structured myofilament lattice. This will be the first use of cardiac myofibrils to study the mechano-chemical alterations produced by point mutations in myosin and actin. The results will be correlated with in vivo and in vitro whole heart ventricular function studies and single molecule mechanics. Our studies will characterize the mechano-energetic alterations in terms of force, velocity, power, work-absorbing and work-producing elements, and cross-bridge kinetic parameters as well as cross-bridge efficiency and economy. We will test the hypothesis that functional alterations resulting from the specific actin and myosin mutations can be segregated into distinct subsets representing those that lead to FHC and those that lead to DCM. The aims are: 1) use our newly developed myofibrillar mechanical assay to assess the extent to which FHC and DCM mutations alter force, velocity, power, dynamic stiffness, and the calcium dependencies of these parameters, and 2) use the skinned strip preparation to assess the extent to which ancillary changes (disarray and fibrosis) contribute to alterations of force, velocity, power, and dynamic stiffness in the mutant lines. This study will address the question as to how mutations in the same molecule can result in such markedly different phenotypes. At a minimum, the phenotypic differences revealed by the various mechanical and biochemical measurements will allow us to assign functional relevance to the underlying structural alterations caused by each mutation. The results of this study will help in our understanding of FHC and DCM.